Past-pm basic string types

[Allison Randal]

Patrick, what's the best way to pass-through string types from a 
compiler to Parrot without doing full string processing? To pass the 
current tests, Punie only needs Parrot's single- and double-quoted 
strings, but Past-pm is escaping them. So:

  print "\n";
reaches the PIR translation as:

  print "\\n"

(I will add full string processing to Punie later, but since other 
compilers will also need basic Parrot string types, it makes sense to 
figure it out now.)

Allison